ABSTRACT Monoclonal antibodies have played a pivotal role in the diagnosis and treatment of cancer for nearly two decades and continue to grow at an exponential pace. Initially developed for their exceptional ability to target tumor antigens and elicit antibody-dependent cellular cytotoxicity (ADCC), they have more recently been used to modulate a patient?s immune system for anti-cancer immunotherapy. While the generation and development of antibodies targeting various cell surface proteins has rapidly progressed, appropriate model systems for pre- clinical testing of such therapeutics has lagged. This is because human antibodies i) don?t fully engage murine or non-human primate Fc receptors (Fc?Rs), ii) are foreign proteins that are rapidly rejected in allogeneic hosts and iii) are often inappropriately tested in immunodeficient xenograft models lacking adaptive immune cells or homologous Fc?R. Thus, our studies have focused on the generation and testing of clinically relevant models to better understand the in vivo activity of diagnostic and therapeutic antibodies. The current proposal aims to now generate and fully characterize novel murine models that allow better preclinical testing of human antibodies by engineering our previously developed humanized Fc?R mouse strains to express human FcRn and IgG1. Expression of human FcRn will allow more accurate pharmacokinetic analysis of human antibodies and assessment of methods aimed at generating antibodies with extended in vivo half-life. By replacing the mouse heavy chain with the constant regions of human IgG1, this model will also allow chronic administration of human IgG-based therapeutics without developing anti-drug antibody responses. By addressing two major hurdles in the field of antibody therapeutics, these models will allow more rapid and efficient pre-clinical toxicology testing and potentially uncover novel mechanisms of Fc-engineered antibodies. Additionally, given the growing interest in immunotherapy, having an immunocompetent model provides an additional advantage over current xenograft models. Finally, as recent data suggest an important role for Fc-Fc?R in radiolabeled antibody diagnostics, these models will provide a clinically relevant model to help improve the development and testing of innovative antibody-based molecules for the in vivo detection and localization of neoplasms.